1. Field of the Invention
The present invention relates generally to the endodontics. More particularly, the invention relates to novel inspection and examination method with which the remaining endurance strength of endodontic files can be inspected non-destructively and equipment for conducting such inspection; said endodontic files are mainly manufactured from stainless steel or superelastic TiNi alloy for purpose of cleaning the diseased portion of root canals.
2. Description of the Prior Art;
The occlusal surface of tooth is formed with coronal body made of hard enamel structure, under which the dentine structure is found. Beneath the dentine structure, there is a pulp chamber and it extends to a root canal toward the root apex. If the dental decay was developed, the caries is initiated by the attack on the enamel surface. Later on, the dental caries will proceed to attack the root canal. The root canal therapy may be defined as the complete removal of the irreversibly damaged dental pulp followed by cleaning, shaping and filling of the root canal system so that the tooth may remain as a functional unit in the dental arch.
Certain principles must be adhered to during the preparation of all root canals; (1) always maintain the original preoperative shape of the root canal and work within the confines of that canal, (2) produce a continuously tapering three-dimension which has its narrowed cross-sectional diameter at the apical constriction, (3) create sufficient width in the canal to permit the use of irrigation in the removal of organic debris and bacteria from the entire canal and to allow adequate space for condensation of the solid-core root canal filling material such as Gutta-percha.
Files are the most useful instruments in the endodontics for a purpose of the removal of hard tissue in canal enlargement. They are manufactured by twisting a blank, which is a square (or sometimes triangle) rod, producing a series of cutting flutes. Mechanical or engine-powered instrumentation has developed into the sonic and ultrasonic fields with a reduction in preparation time. What has improved is the irrigation and concurrent removal of toxic debris from the root canal.
An endodontic file has normally two functions; they are filing action and reaming action. If the filing action is used, the flutes scrape against the walls, gouging a portion of dentin and removing it from the canal. If reaming action is used, clockwise turning should be employed. The flutes contacting the walls scrape and shave the dentine to widen the preparation.
Due to complexity and irregularity of root canal morphology, the filing action and reaming action make the complete endodontic therapy more complicated and challenging. In order to accommodate to such complicated and irregularly shaped root canals, the endodontic file should be very elastic, particularly the tip portion thereof. Furthermore, there is a variety of size in terms of tip diameter and stem diameter, and length as well.
Recently, TiNi material has been extensively used to fabricate the endodontic files because these alloys possess unique characteristics of superelasticity. Superelastic property reduces the chance for procedural accidents such as zipping, ledging and transportation of files. While the superelastic properties of TiNi instruments are highly desirable, instrument breakage within the canal can still occur. It is said that if the endodontic file was broken and left in the portion of the root canal, it is not the malpractice. But, if the endodontist fails to record this evidence on the patient's chart and fails to mention this incidence to his or her patient, it is the malpractice.
M. L. Zuolo et al. observed that the endodontic files were failed due to various mechanical reasons; they may include a rapid wear, breakage, gouges and pits on flutes, and twisting on shaft [Journal of Endodontics, Vol. 18, p.336; 1992]. Moreover, J. P. Druett et al. investigated the low cycle fatigue damage process on superelastic TiNi endodontic files [Journal of Endodontics, Vol. 23, p. 77; 1997]. They found that all tested TiNi files were fatigue-failed and number of cycles to failure depended on the curvature angle (which indicates how sharp the root apex is turned and shaped).
If the file is evaluated to be non-damaged, it will be sterilized and re-used (or recycled) to the next patient. The normally practiced sterilization is an autoclave-sterilization at 120.degree. C. for 30 minutes at 20 psi pressure. B. F. Mitchell found that the stainless steel files have shown a decrease in fracture resistance following repeated autoclave sterilization [Oral Surgery, Vol. 55, p.204; 1983]. On the contrary, E. P. Kiss et al. observed that regardless of the size of TiNi rotary files, multiple (five) cycles of autoclave sterilization resulted in a significant increase in torsional fracture resistance [Journal Dental Research, Abstract 547; 1997]. However, Y. Oshida observed that the progressive damage on TiNi files was found to be dominant over the recovery process by the autoclaving (if any) [unpublished data; 1998].
As described in the above, although the breakage and leaving the portion of endodontic file in the root canal is not malpractice, it will cause many problems to certain types of patients. If the patient is hypersensitive to nickel element, he or she will be suffering from the nickel allergic reaction. For a more serious case, the dissolved nickel element will act as a carcinogenicity. Because nickel content in the TiNi (about 50 weight %) files is higher than that in stainless steel (about 8 weight %) files, the above potential problems will be worsened when the superelastic TiNi files are broken and left behind the root canal portion. Although the biocompatibility of TiNi implant was well documented, it is based on the in vitro studies using the artificial saliva, which has a chlorine concentration with about seven (7) times dilute than that of dentinal liquid. Accordingly, it can be easily speculated that the dissolution of nickel element out of the broken TiNi would be much higher than the case in the intraoral environment.
Even with these potentially harmful element containing materials, there is no scientific standards to assess the cumulative damage on TiNi or stainless steel files before the mechanical breakage. What is normally practiced with the endodontics is that the degree of bending of the tip portion of files and cutting efficiency of used files are major two parameters for the endodontist to evaluate the progressive damage and judge as to whether or not it can be recycled after the autoclave-sterilization or should be abandoned. These observation is normally performed by either naked eyes or under relatively low magnification lens (for example, .times.20). Hence these are the macroscopic inspection and limited only on the surface layers.
In light of this background, there remain needs for novel endodontic file inspection method that are more reliable to an endodontist and safe to the endodontic patient and for fabricating the equipment for performing these novel inspection on endodontic files. The present invention addresses these needs.